Fix bug in source verification on Windows
The way to pass the set of directories to be used for validating the
source caused issues on Windows systems, as it replaced ":" characters
to change from strings to lists in CMake.
This caused the top level directory to be changed from e.g. "D:" to
"D;", messing up the rest.
Fixed the message passing to only pass the final directory, and modify
to full path after the set has been passed through.
Fixes #3493
Change-Id: I4076b8e87dc8d72a83af6aa0c7ab4b3366917ed0